The present invention relates to the art of neuroanatomy, and, in particular, is directed to an improved means for detecting neural circuits. The present invention has resulted in the course of work conducted with funds provided, in part at least, by an award or grant from the National Institute of Health (NIH).
A severe limitation of known neuroanatomical methods known today is that they can only examine the connection between two groups of neurons at one time. Since, however, even simple reflexes result from interactions between three or more neurons, a detailed understanding of the basic neural circuitry is not readily performed by present methods.
Accordingly, efforts have been made to provide a neuroanatomical pathway tracer which will cross more than one neuro junction and which can be readily detected. Recent studies on tetanus toxin have demonstrated that this substance moves retrogradely (i.e, backwards) across synaptic junctions, Schwab and Thoenen, "Selective Transsynaptic Tetanus Toxin After Retrograde Axonal Transport in Peripheral Symphetic Nerves," BRAIN RESEARCH 122: 459-474 (1977). Thus, this tracer can reveal not just connected pairs of neurons, but also connections between three or more neurons in a single experiment. For example, when tetanus toxin is introduced to neuron A, the tracer moves backwards to neuron B which terminates on neuron A. Then the tracer continues backwards through the nervous system to label the neuron C which, in turn, synapses on neuron B, thereby identifying the neural circuit C-B-A. This feature provides a tremendous tool in determining neuro anatomical circuitry. There are, however, two serious problems regarding the use of tetanus toxin in the nervous system. Tetanus toxin is highly toxic, and the toxin can only be crudely localized by conjugation to I.sup.125, which is highly radioactive.
Thus, it is an object of the present invention to provide an effective neural pathway tracer which can be readily localized for analysis, while at the same time, providing a safe tracer agent which can be used without adverse side effects to the host.